When designing high voltage transformers, it is important to maintain adequate distance between the high voltage components in the transformer and the low voltage components in the transformer in order to prevent shorts. A cross-section of a typical high voltage transformer having multiple secondary windings is shown in FIG. 1. At its simplest level, high voltage transformer 20 includes a two-piece ferromagnetic core 22a and 22b, primary windings 24, a plurality of secondary windings 26a, 26b, . . . 26f, and insulation 28. The primary windings are constructed of a continuous conductive wire that is wrapped around the two legs of the core a desired number of turns. Similarly, the plurality of secondary windings 26a, 26b, . . . 26f are each constructed of a continuous conductive wire wrapped around a cylindrical nonconducting spool (not shown) a desired number of turns. The plurality of secondary windings surround the primary windings and the core, but are spaced a distance away from the primary windings by intervening insulation 28. As is well known in the art, an alternating voltage applied across the primary winding will be stepped up by the transformer proportionally to the ratio of the number of turns in each secondary winding with respect to the number of turns in the primary winding. The transformer in FIG. 1 also contains a plurality of diodes 30a, 30b, . . . 30f that are integrally formed with the transformer and connected in a bridge configuration across the output of a corresponding secondary winding 26a, 26b, . . . 26f. A stepped-up and rectified voltage is therefore produced by each secondary winding/rectifier pair.
While the transformer configuration shown in FIG. 1 may be readily adapted for low-voltage applications, when the configuration is used for high voltage applications (in the kVolt range), several shortcomings of the design become apparent. One disadvantage of the traditional design is that design tolerances in the transformer become very critical. When voltages in the kVolt range are being generated by transformer 20, a first distance 32 between the primary and secondary windings, as well as a second distance 34 between the windings and the external surface of the transformer must be carefully controlled. If the amount of insulation 28 is insufficient for the voltage applied across the insulation, the insulation may break down causing the primary winding to short to the secondary winding and leading to a transformer failure. Similarly, if the amount of insulation between the secondary winding and the external surface of the transformer is insufficient, a short may occur from the secondary winding to a point outside the transformer. In addition to potentially damaging the transformer, the external short could put individuals or other equipment near the transformer at risk.
Moreover, in high voltage applications, the insulation in the transformer has a tendency to degrade due to the extreme swings in the generated output voltage. Insulation 28 contains dipoles that will align themselves with the alternating electric field generated as current flows through the winding. The application of an alternative current (AC) voltage across the winding therefore causes the dipoles to twist in response to the changing polarity of the input signal. The twisting motion of the dipoles is more pronounced the higher the generated output voltage. In high voltage applications, the twisting of the dipoles heats the insulation and causes the insulation to degrade. As the insulation degrades, the resistance of the insulation falls and increases the likelihood that a short will occur. Ultimately, the performance of the transformer is compromised and the transformer must be replaced.
Another disadvantage of the traditional transformer design is that in high voltage applications, the secondary winding of the transformer typically must have a large number of turns in order to step up the voltage to the desired amplitude. As the number of turns in the secondary winding increases, so does the reflected capacitance of the secondary winding. Those skilled in the art will recognize that to efficiently drive a large capacitive load, it is desirable to place an inductor in series with the load. As the reflected capacitance of the secondary winding of the transformer increases, it is therefore necessary to include a progressively larger inductor in the system incorporating the transformer. In addition to becoming prohibitively expensive, the inductor will increase the size of the system incorporating the transformer.
The present invention is directed to a transformer construction for high voltage applications that overcomes or minimizes the above-mentioned disadvantages.